Simplify Layout Transition State #trivial

[Adlai-Holler]

• Create a formal asdispalynode_iscancelled_block_t for tracking cancellation of layout transitions.
• Use atomic variables for transitionID and pendingTransitionID, and take them out from under __instanceLock__.
• Remove the BOOL transitionInProgress field. Instead, we reset transitionID to Invalid if there is no transition in progress.
• Use a global autoincrementing int for transition IDs, rather than a separate autoincrement per-node.
  • Could this cause overflow? No, not any time soon. We support up to 2.1 million transitions which should be plenty. We can change the data-type for our transitions in a separate diff if we want to.

[ghost]

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